Material’s secrets revealed with ultrasound scans of elasticity matrix

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Researchers at Nottingham University have developed SRAS, a new technique that evaluates a material’s elasticity matrix by measuring the speed of sound across the material’s surface.

elasticity matrix
Diagram of SRAS laser ultrasound system: The high-energy pulse of light from the laser creates a sound wave that travels along the surface of the material. Precisely measuring the speed of this wave allows the crystal orientation and elasticity to be measured (Image: Nottingham University)

Claimed to be a world first, the innovation uses high-frequency ultrasound to produce microscopic resolution images of the microstructure and maps the relationship between stresses and strains in the material (the elasticity matrix). By precisely measuring the speed of sound across the surface of these crystals, their orientation and the elasticity of the material can be revealed.

According to Nottingham University, this EPSRC-funded technology is starting to be used in fields such as aerospace to understand the performance of new materials and manufacturing processes. The advance will also launch a new field of research as the technique is used as a completely new way to evaluate materials for improving safety in systems such as jet engine turbine blades, or in developing new designer alloys with tailored stiffness.

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In a statement, Paul Dryburgh, co-lead on the study from the Optics and Photonics Research Group at Nottingham University, said: “Many materials are made up of small crystals. The shape and stiffness of these crystals are essential to the material's performance. This means that if we tried to pull on the material, as we would a spring, the stretchiness depends on the size, shape, and orientation of each of these hundreds, thousands or even millions of crystals. This complex behaviour makes it impossible to determine the inherent microscopic stiffness. This has been an issue for over 100 years, as we’ve lacked an adequate means to measure this property.”

“The development of SRAS++ is a notable breakthrough because it provides the first method to measure the elasticity matrix without knowing the distribution of crystals in the material,” added co-author, Professor Matt Clark - also from the Optics and Photonics Research Group. “SRAS doesn’t require exacting preparation of a single crystal; it is fast and offers unparalleled measurement accuracy. The speed of the technique is such that we estimate that we could repeat all the historical elasticity measurements of the past 100 years within the next six months.”

Previously, the only way to measure the elasticity matrix was to cut up the component or grow a single crystal of the material, a process that cannot be done for many materials, including titanium alloys used in jet engines. Estimates are that less than 200 materials out of many thousands have had their elasticity measured. Consequently, the elasticity of most industrial materials is unknown, with significant - and potentially hazardous - uncertainty in the performance of the material put to use.

Laser ultrasound allows ultrasound to be created in an area measuring 200µm. By measuring the speed of sound across each crystal, the researchers can tell the shape of the crystals and the elasticity matrix of the material at a microscopic scale. Sound travels across the surface of metals 10 times faster than through air (at ~3000m/s).

The team’s findings are reported in a new paper, entitled ‘Measurement of the single crystal elasticity matrix of polycrystalline materials', published in Acta Materalia.